The invention relates to a piston for closing or sealing a cartridge, namely a piston for insertion into the cylindrical container part of a cartridge which serves for holding a flowable material and which comprises a cylindrical sidewall and an end wall.
Cartridges are intended for holding a flowable material and are used, for example, wherever a possibly moisture-sensitive, flowable material has to be applied in portions in grooves, joints and the like. The cartridges used in building and in industry are produced today in particular from aluminium. They have a cylindrical container part with an outlet nozzle arranged at the front end and a piston inserted into the orifice in the rear end and movable in the longitudinal direction of the cartridge and intended for sealing the interior of the cartridge and ejecting the flowable material. The material is usually ejected by using a hand-operated injection gun with a plunger which is tailored to the piston and moves the piston towards the outlet nozzle when the cartridge is used.
In a known method for closing filled cartridges, the inner surface of the cylindrical container part is provided, in the region of the rear end section, with an all-round wax layer serving as a sealant, and subsequently a piston whose external diameter is smaller than the internal diameter of the container part is inserted into the container part. This has the advantage that, when the piston is inserted, the air present in the interior of the cartridge and displaced by the piston can readily escape outwards, but, on the other hand, the disadvantage that for a fluid- and gas-tight seal, the piston inserted into the container part must subsequently also be pressed, i.e. expanded, by the container part, which is usually effected by means of an expanding device specially provided for this purpose.
It is the object of the invention to provide a novel piston for cartridges of the above-mentioned type which makes it possible to simplify the fluid- and gas-tight sealing of the cartridge in such a way that it is possible to dispense with the expansion, i.e. with the additional use of an expansion device.
This object is achieved by a piston, which comprises a cylindrical sidewall (8), which is at least as large as the internal diameter of the cylindrical container part of the cartridge, wherein the cylindrical sidewall is provided with at least one bevel starting from the end wall and dimensioned in such a way that it can be pressed with a tight fit into the container part and that, when the piston is pressed in, the at least one bevel, together with the cylindrical wall of the container part briefly forms a passage through which the air displaced by the piston can escape outwards.
The novel piston, also referred to below as press piston, is distinguished essentially by the fact that it is in the form of a cylinder which is closed at one end and whose external diameter is also at least as large as the internal diameter of the cylindrical container part. The piston according to the invention is thus dimensioned so that it already has a tight fit when inserted into the container part. Although this is not possible without additional application of force, it can readily be effected by means of a conventional, axially operating press device. The advantage of this procedure is obvious. Thus, it is possible to dispense with the subsequent expansion of the piston by means of a device specially provided for this purpose.
In addition, the cylindrical sidewall of the piston according to the invention is provided with at least one bevel starting from the end face, so that, when the piston is pressed into the container part of the cartridge, the at least one bevel, together with the cylindrical wall of the container part, briefly forms a passage through which the air displaced by the piston can escape outwards, and that consequently no excess pressure adversely affecting the tightness of the cartridge is generated in the interior of the cartridge and in particular no residual air remains in the cartridge and triggers a chemical reaction, for example undesired hardening of the flowable material inside the container part.
The press pistons according to the invention preferably consist of metallic material, for example of tin plate or aluminium.
It is expedient to provide at least two, preferably four or six, bevels arranged symmetrically with respect to one another. Such an embodiment of the press piston according to the invention leads to optimum venting of the cartridge when the piston is pressed into the container part, and does so with constant moisture and sealing properties so that the shelf-life and operational safety of cartridges sealed by means of press pistons according to the invention is ensured.
The bevels according to the invention do not cover the total circumference of the cylindrical sidewall and arexe2x80x94as is evident from the drawing explained belowxe2x80x94essentially dent-like recesses in the cylindrical sidewall.